Languishing in solitary confinement in Zimbabwe's maximum security Chikurubi prison in March, Simon Mann was getting desperate. The Old Etonian and former SAS officer had been arrested at Harare airport two weeks earlier along with a plane load of mercenaries after landing to pick up a consignment of AK-47 rifles, mortar bombs and 75,000 rounds of ammunition.

The men on board the Boeing 727-100 had allegedly been on their way to mount a coup in Equatorial Guinea, a small, malarial country in west Africa ruled by a tyrant but newly and filthily rich in offshore oil.

Instead of a coup amid untold riches, Mann found himself staring down the barrel of a long prison sentence - or even execution if an extradition request from Equatorial Guinea was successful. So he penned a letter on scraps of paper to his wife and lawyers, demanding that they get people on the outside to exert both their money and influence to get him released.

But by writing the letter - a copy of which has been seen by the Guardian - he linked what had at first seemed to be little more than a doomed Boy's Own adventure in a forgotten corner of west Africa to a coterie of rightwing businessmen with links to the highest echelons of the British establishment.

Scrawled over two plain pages and a scrap of magazine, Mann's letter referred to a contact called "Scratcher" - Mann's nickname for Sir Mark Thatcher, son of the former Tory prime minister and perennially controversial businessman.

When the note was intercepted by the South African intelligence services on its way out of the prison, a train of events was set in motion that led yesterday to the raid on Sir Mark's Cape Town home.

"Our situation is not good and it is very URGENT," Mann wrote. "They [the lawyers] get no reply from Smelly and Scratcher [who] asked them to ring back after the Grand Prix race was over! This is not going well."

But Mann then went on to suggest that Scratcher's involvement amounted to more than using his contacts to lobby for their release.

"It may be that getting us out comes down to a large splodge of wonga! Of course investors did not think this would happen. Did I?" he wrote. "Do they think they can be part of something like this with only upside potential - no hardship or risk of this going wrong. Anyone and everyone in this is in it - good times or bad. Now its bad times and everyone has to F-ing well pull their full weight."

He left what would appear to be the most incriminating detail to last: "Anyway [another contact] was expecting project funds inwards to Logo [Mann's firm] from Scratcher (200) ... If there is not enough, then present investors must come up with more."

While the letter certainly suggests Mann was expecting Sir Mark to make a $200,000 (£111,000) investment, he does not specify whether it was for the coup.

The letter also refers to David Hart, the former Old Etonian millionaire adviser to Lady Thatcher during the miners' strike. "We need heavy influence of the sort that ... Smelly, Scratcher ... David Hart and it needs to be used heavily and now," Mann wrote.

Even the disgraced Tory peer, Lord Archer, has been dragged into the controversy after $134,000 (£74,000) was deposited into Mann's bank account in the name of JH Archer four days before the coup attempt. Lord Archer categorically denied any involvement in the coup.

Ever since the coup plot was alleged at Harare airport on March 7, there have been murmurings about Sir Mark's involvement. He and Mann were close friends who regularly had dinner together, and both owned substantial properties in Constantia, the secluded suburb of Cape Town popular with rich expat Britons.

Mann is a complex character, part buccaneering thrill seeker, part businessman, who left the SAS to make a living fighting wars in Africa. It is easy to see how Sir Mark - whose demeanour would suggest he would like to be viewed as something of an adventurer himself - might be attracted to the former SAS officer.

"Nobody is denying they are close friends - and they have been friends for a long time," Sir Tim Bell, Lady Thatcher's former PR adviser and now informally advising her son, said yesterday. "I have not spoken to him at all at any point since this started about six months ago. He has studiously avoided discussing the issue."

Greg Wales, another man with alleged links to the coup plot and a long-standing friend and former business partner of Mann's, told the Guardian yesterday: "Simon and Mark did a number of business deals together - in mining, and aircraft and fuel brokerage. The police would have found a lot of stuff on these matters."

But it remains unclear what - if any - evidence the South Africans have to tie Sir Mark directly to the coup, beyond Mann's letter. There have been rumours that he may have made an investment in Mann's Logo Ltd company through a South African company called Triple A Aviation, which in January signed a contract with Mann's Logo company to provide aircraft and aviation services.

According to his lawyer yesterday, Sir Mark was arrested on suspicion of providing financing for a helicopter linked to the coup plot.

Banking records show the company, which trades as Air Ambulance Africa from the town of Bethlehem in the Free State, paid $100,000 (£55,000) into Logo's account on March 2, less than a week before the coup attempt was launched. The head of Air Ambulance, Crause Steyl, did not return calls yesterday. His brother Niel, a former pilot for the infamous South African mercenary outfit Executive Outcomes in the 1990s in which Mann was also involved, was the pilot of the ill-fated Boeing that landed at Harare.

According to well-placed South African sources, Triple A provided a twin-engined King Air turboprop which flew the exiled Equatorian Guinea opposition leader, Severo Moto, from Spain to Bamako in Mali on the eve of the alleged coup attempt, in preparation for his triumphant return to power.

Friends of Sir Mark in South Africa, however, claim that he had entered into a completely separate contract with Triple A to provide an air ambulance helicopter for work in Equatorial Guinea. "I don't think he knew what he was getting into," one told the Guardian.

The genesis of the alleged coup plot, according to Mann's own witness statement, began in January 2003 when he was introduced to Eli Calil, a Chelsea-based businessman, in London - a friend and financier of Mr Moto, leader of the Party for Progress of Equatorial Guinea and president of the Guinean government in exile in Madrid.

Mr Calil has denied any knowledge or involvement in the coup and his lawyers have suggested that the written and verbal confessions of Mann and his alleged co-conspirator South African arms dealer Nick du Toit, currently in trial in Malabo, were extracted through torture.

But Mann wrote in his signed statement after his arrest. "Ely Calil asked me if I would like to meet Severo Moto... I met Severo Moto in Madrid. He is clearly a good and honest man. He had studied for the priesthood ... At this stage they asked me if I could help escort Severo Moto home at a given moment while simultaneously there would an uprising of both military and civilians against Obiang ... I agreed to try and help the cause."

Preparations for the coup - money, men, logistics and a suitable plane - were soon set in motion by Mann through two companies based in Guernsey, Logo Ltd and Systems Design Limited. Mann himself sold some of his shares and put in $400,000 to cover the cost of a specially converted Boeing 727 which was bought a week before the coup attempt from a firm in Kansas. Guardian inquiries have established that the aircraft had been converted for US military use so that it could take off and land on shorter runways. It also had a pressurised cargo hold which could be accessed during flight.

The final stages were completed in February. Using his military and arms dealing contacts, Mr du Toit helped to recruit the mercenaries - apartheid-era soldiers in South Africa - and to introduce Mann to the head of the Zimbabwean Defence Industry in Harare for the weapons.

The broad plan, according to Mr du Toit's account, was for the plane to pick up the 64 mercenaries at Wonderboom airport near Pretoria and then fly on to Pietersburg international airport to clear customs for Harare. In Harare the plane would refuel and pick up the arms - 150 hand grenades, 80 60mm mortar bombs, 100 RPG-7 anti-tank projectiles with 10 launchers, 20 light machine guns, 61 AK-47 assault rifles and 75,000 rounds of ammunition.

From there the plane should have flown straight to Malabo and landed at 2.30am on Monday March 8, with Mann in Harare keeping in touch with Mr du Toit in Malabo on his satellite phone. Once the mercenaries had landed one team was designated to secure the airport. The rest were to be driven into town with guides and vehicles provided by Mr du Toit.

While separate teams set up road blocks to prevent the military leaving their bases and moving into town, another group would capture minister Antonio Javier - Mr du Toit's business partner - who would guide them to the sleeping president. The president and brother Armagol would then be taken to the airport and, "if not killed in this operation", would be flown to Spain.

Meanwhile Mr Moto would have landed at Malabo airport, 30 minutes after the mercenaries. He would "call some supporters he claimed to have within the military and ask them to take control of the security situation". By sunrise the people of Equatorial Guinea would hear on the radio and see on television their new leader.

But the plot, if that is what it was, could not have gone more spectacularly wrong - reinforcing rumours of an intended coup circulating in special forces circles in Pretoria and even openly discussed at an academic meeting about oil, with US and Foreign Office officials present, in London.